This invention relates to headphones designed primarily for classroom use in the education environment. A great variety of audio and audio-visual equipment is now commonly employed for instructional purposes in school classrooms. The use of headphones for students to listen to instructional audio material is often desirable in order to provide a noise-free private environment for individual and small group instruction. These audio sources include study carrels, phonographs, tape players, compact disk players, video displays, film projectors, and computers. The audio sources commonly employed in the education environment may be monaural or stereo and are often accessed by a single jack that accepts only a xe2x85x9-inch or a xc2xc-inch headphone plug. This has resulted in difficulty for teachers and equipment custodians to stock and maintain control of headphone equipment for use with all of these various audio sources.
Existing headphone designs for accessing the various audio sources described above are shown in FIG. 1 and FIG. 2. FIG. 1A discloses a headphone 10 that includes left and right earpieces 12 and 14 respectively, a connection cord 18 terminated in a stereo plug 16 and an xe2x80x9cin-linexe2x80x9d switch 20 to select a stereo or monaural source. FIG. 1B is a circuit diagram showing the switch 20 set for a stereo source. It shows a common connection from the plug sleeve-contact 22 to like identified terminals (+) of the two earpiece drivers 13 and 15. The plug ring-contact 24 is connected to the other identified terminal (xe2x88x92) of the left earpiece driver 13 and the plug tip-contact 26 is connected to the corresponding identified terminal (xe2x88x92) of the right earpiece driver 15. This setting provides normal stereo listening from a stereo source. FIG. 1C shows the circuit diagram with the switch 20 set to access a monaural source. There is no connection to the plug 16 ring-contact 24. The plug 16 tip-contact 26 is connected to the (xe2x88x92) terminals of the earpiece drivers 13 and 15. This allows a monaural source to be heard in both ears. A plug adapter 28 provides access to audio sources that accept only a xc2xc-inch plug.
Although a headset with a mono/stereo selection switch would require only the addition of a single stereo plug adapter to change the headset plug size, the use of such a switch for selecting a monaural or stereo source has not found favor in the education environment for the following reasons:
1. The selection switches are likely to be played with by the students and have not proved to have the ruggedness and reliability required for classroom use, and;
2. The source must be positively identified prior to making a switch selection.
3. Teachers prefer to configure the headset for the proper source when used by younger children so they cannot easily change it to the wrong configuration.
FIG. 2 discloses additional prior art for a headphone system that can access either a monaural or stereo audio source that accepts only a xc2xc-inch or xe2x85x9-inch plug. FIG. 2A shows a stereo headset 30 consisting of an adjustable headband 32, left and right earpieces 34 and 36 respectively, connection cord 38 terminated in a molded plug assembly 40 that includes a xe2x85x9-inch stereo plug 42. The plug 42 includes external screw threads 58 that mate with internal screw threads of three plug adapters 44, 46 and 48. A plastic holder 50 for the plug adapters is secured to the connection cord 38 to store them when not in use. These plug adapters are shown enlarged in outline drawings in FIG. 2C. FIG. 2B is a schematic diagram of the headset for accessing a stereo source with the xe2x85x9-inch stereo plug 42. The ring-contact 54 is connected to the (xe2x88x92) terminal of the left earpiece driver 35 and the tip-contact 56 is connected to the (xe2x88x92) terminal of the right earpiece driver 37. A connection to the (+) terminal of each earpiece driver is made to the sleeve-contact 52 of the plug 42.
Plug adapter 44 shown in FIG. 2C converts the headset 30 for listening to a monaural source equipped with a xc2xc-inch output jack. It connects the tip-contact 60 to both the tip-contact 56 and to the ring-contact 54 of the stereo plug 42 shown in FIG. 2B thus allowing the audio source to be heard at both earpieces 34 and 36 of the headset. Sleeve-contact 62 connects the source sleeve circuit to sleeve-contact 52 of plug 42. Similarly plug adapter 46 provides for listening to a monaural source equipped with a xe2x85x9-inch output jack. It connects the tip-contact 64 to both the tip-contact 56 and to the ring-contact 54 of the stereo plug 42 shown in FIG. 2B thus allowing a monaural audio source equipped with a xe2x85x9-inch output jack to be heard at both earpieces 34 and 36 of the headset.
Plug adapter 48 shown in FIG. 2C adapts the xe2x85x9-inch stereo plug 42 of the headset to a stereo source equipped with a xc2xc-inch stereo jack. It connects the tip-contact 68, the ring-contact 70, and the sleeve-contact 72 to their respective contacts 56, 54 and 52 of plug 42 thus providing for normal stereo listening to the source.
The headphone design of this invention accesses a monaural or stereo source with a connection cord preferably terminated in a xe2x85x9-inch stereo plug. In a first embodiment of the invention the right and left signals from a stereo source are connected in a normal manner directly to each earpiece driver; that is, the right channel is connected to an identified terminal of one (usually the right) earpiece driver, and the left channel is connected to a similarly identified terminal of the other earpiece driver. A common connection from the stereo source is connected to the other terminal of each earpiece driver. In this embodiment of the invention a connection is also made between the right and left channel identified terminals of the earpiece drivers through an impedance element. The magnitude of the impedance element is chosen with respect to the impedance of the earpiece drivers such that an audio signal appearing at only one earpiece driver terminal, as would be the case with a monaural source, is coupled to the other earpiece driver with a reduction in signal strength at the other earpiece driver terminal of the order of two decibels. This slight reduction in loudness in one ear is virtually imperceptible to the listener, probably aided by a psycho-acoustical effect that masks the slight reduction in loudness in one ear if the other ear suffers no reduction in loudness.
The impedance element results in a slight mixing of right and left channel audio signals from a stereo source. The amount of channel mixing described above is so small that stereo separation is virtually unaffected and no reduction of stereo imaging is perceptible to the listener. The reason for this small amount of channel mixing is the typical low impedance of the audio sources compared to the value of the impedance element; a ratio of about one hundred. This results in the crosstalk between channels caused by the impedance to be about 40 decibels below the level of either channel, a level well below the channel separation requirement for good stereo performance.
In a second embodiment of the invention the stereo channel that is connected to the ring contact of the stereo plug is connected directly to one earpiece driver and the other channel that is connected to the tip of the stereo plug is connected to each earpiece driver through a an impedance element. A monaural source is always accessed by the plug tip, and thus each earpiece driver is connected to the source through one of the impedance elements. If each impedance element is one-half the value of the impedance element used in the first embodiment described above the monaural signal will be heard at each earpiece with equal loudness at a level of the order of one decibel less than if a direct connection to each earpiece driver had been made as in a conventional stereo headset. With a stereo source one channel will be heard with no reduction in loudness compared to a conventional stereo headset and the other channel will be heard with reduced loudness of the order of one decibel. This slight difference in the level of the stereo channels is not perceptible and can be easily balanced, if desired, by a balance control available with many stereo sources. Again, because of the typical low impedance of the stereo source, the stereo crosstalk due to the coupling impedance elements is about xe2x88x9234 decibels, providing excellent stereo performance.
Thus the headset can be used with either a monaural or stereo source without requiring any action to identify the nature of the source or configure the headset for the type of audio source, while providing virtually the same efficacy of use as a separate headphones dedicated to either a monaural or stereo source, or a headphone provided with a mono/stereo switch selector, or a headphone employing stereo-to-monaural plug adapters.
Accordingly, it is an object of the invention to provide a headphone that can be used for automatic listening to a monaural or stereo audio source, that does not have to be configured for the type of audio source, and that provides for hearing with substantially equal loudness at both earpieces either a monaural signal or equal magnitude right and left channel stereo signals and with which the stereo source is heard with right and left channel separation and stereo imaging substantially as afforded by the stereo source.
It is a further object of this invention to provide at a lower cost than heretofore possible a headphone that requires no switching means and only one plug adapter to access the four configurations of sources represented by a monaural or stereo source with either a single xe2x85x9-inch or single xc2xc-inch output jack.